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Thread: Triangular stars

  1. #11
    AustinPSD's Avatar
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    Default Re: Triangular stars



    Gravity holds the mirror in place in the cell. The mirror 'floats' within the primary cell, and radial or pressure at its circumference creates strain within the mirror, which in turn distorts the mirror's figure. The collimation adjustment posts (at the opposite ends of the collimation adjustment screws) adjust the mirror's tip/tilt, while the edges of the posts the clips are attached to maintain the mirror's optical center coaxial to the primary cell. Any pressure, particularly on the mirror's edges will distort the mirror's figure, causing things like the triangular stars.
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  2. #12
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    Default Re: Triangular stars

    Oh, I'm not trying to disagree with the knowledge of experienced astronomers - I really appreciate the advice that people have given me and I'm just trying to understand for myself why I'm doing what I'm doing. I don't like blindly making adjustments to expensive things (telescopes, cars, computers, cameras, etc) without knowing what I'm doing. Sorry if I came across as impertinent; I'm just inquisitive.

    So you're saying that the mirror is held in its cell only by gravity, and will stay put provided the telescope is pointed mostly upwards. What about when I remove the telescope from its mount, put it in its case and sling it in the boot of my car? At some point it is likely to end up pointing downwards. If the mirror slumps forward to rest on the clips and later falls back onto the cell, what guarantee is there that the mirror will return to the same place? Is this the reason that people recommend collimating a Newtonian before every use?

    Thanks for your patience
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  3. #13
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    Default Re: Triangular stars

    The gap between the edge clips and supports is intended to be small, so the amount of movement in any direction is consequently very small. The mirror is concentric, and generally immune to any effect of rotation (which is rare) around the opto-mechanical center of the primary cell and telescope tube. The foam pads on the edge clips do two things; they prevent direct metal-to-mirror contact to minimize edge chips, and the friction created between the foam and mirror surface discourage rotation in the cell.

    The amount of movement possible when the system is properly adjusted is very small, less than 1mm in general, in any direction.

    When transporting a Newtonian reflector, you will generally need to collimate it, particularly if it is not handled very gingerly.

    Here at McDonald, our telescopes do not use edge clips - if the telescope is moved to a position below the horizontal, the mirror can actually tip out of the cell. Try that with a 107"/2.7M mirror... not for the feint of heart.
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  4. The Following User Says Thank You to AustinPSD For This Useful Post:

    dj_judas21 (11-15-2013)

  5. #14
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    Default Re: Triangular stars

    Thanks AustinPSD and KT4HX for your extremely helpful answers. I learned something, and it didn't even hurt! Tonight I'll re-adjust my edge clips, re-collimate, wait for the clouds to pass before and have another go at using the telescope.

    Enjoy your weekend!
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    Default Re: Triangular stars

    The primary will probably slide back and forth a little bit within the mirror cell. How much depends on the foam bumpers within the mirror cell and it should only be a couple of millimeters.
    When I first got my 10 inch, the primary clips were so loose that I could push on the back of the primary and move it about 1/2 inch. The weight of the mirror would make it reseat itself and since I wasn't having any issues with my views I left it that way until tree pitch and bat poop forced me to try and clean my primary. After doing some research on here and a couple other sites on cleaning the mirrors, I found out that my primary clips were much looser than they should be. My first attempt at tightening the clips was to have them barely touching which resulted in my defocused stars having a slightly square shape (my scope has four primary clips). The next attempt was after asking some advice on this site and spacing them with a business card which resulted in my defocused stars being round again.
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  7. #16
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    Default Re: Triangular stars

    First, thanks Austin, you said what I was trying to think of, but couldn't find the right word - gravity! Second, DJ, no problem, we know you're just trying to get a handle on what is going on. The only way to do that is keep asking questions until you get the answers you need to clear it up for you.

    I will say that every time I take my Z10 out, I always check the collimation. That is a good practice to get into. While in my case, I carry it (two trips) from the garage around the house to the backyard. It doesn't get banged around hardly at all, but I still always check it. I find that I have to tweak it just a little usually. It never gets off by much. Even when we take it over to the mountains (about 2 to 3 hours drive) in the van, it doesn't get out of collimation by much, just requiring a little tweak. Now the 17.5 inch is a different story. Its a truss style, so it gets assembled and disassembled each deployment, and of course it has to undergo more alignment each time for that reason.
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  8. #17
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    Default Re: Triangular stars

    If you're not having luck with the sky to get your scope fixed, you might have better luck using one of the optics test images. Do a google image search for "USAF 1951," for example, and you'll see what I mean. I'd include a few but I'll have to consult the rules first, I think I can't post links or anything like that. When you image search on google, be sure to click on "search tools" under the search bar so that you choose a size "larger than" 4MP, and you'll get a nice list of test images for cameras, riflescopes, telescopes, etc. Some charts such as the "Edmund Scientific Company - Resolving Power Chart" includes the USAF 1951.
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  9. #18
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    Default Re: Triangular stars

    Quote Originally Posted by Jason Dee View Post
    If you're not having luck with the sky to get your scope fixed, you might have better luck using one of the optics test images. Do a google image search for "USAF 1951," for example, and you'll see what I mean.
    Thanks for the tip. I already have a large USAF 1951 chart printed on rigid board at home, which I use for testing old photographic lenses. (My other main hobby is collecting, restoring and using old cameras). I hadn't thought of using it for testing the telescope.

    Although probably the best bet for collimating a telescope is a distant point source of light. I could probably find a distant streetlight visible from my garden if the sky is cloudy. I've never really tried using my telescope in the garden; I live in a light-polluted city so I always drive to the countryside when I want to observe.
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    Default Re: Triangular stars

    Well, I spent some more time this evening working on my scope. I slackened off the primary mirror clips so they are no longer touching the mirror - there's enough room for a business card. I re-assembled, collimated very quickly using a laser collimator and found a bright star. Extremely disappointed to find that my stars are still triangular, although perhaps a little less so.

    I wondered if it might have something to do with the secondary, which also has three screws. After some experimentation, it seems that the secondary is not to blame.

    So what else might cause this problem? The primary mirror clips are not touching the front surface of the mirror but they are very, very close to the edges of the mirror. I guess not much can be done about this without buying a larger mirror cell, or a smaller mirror.
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  11. #20
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    Default Re: Triangular stars

    There is another, somewhat remote possibility - coma defects.

    This is less obvious to me, as you indicate using a coma corrector, and your telescope is an f/5 instrument.

    I think a couple of things come to mind:

    - have you tried with, and without the coma corrector? Coma normally is not an issue in optical systems above f/4.1.
    - what happens when you orient the components differently (i.e. rotate them about the center of the optical axis)? Does the shape of the defect change, or just move to a different position without a shape change?

    A coma defect can be the result of collimation, a fast optical system (<f/4.1), exacerbated by other conditions, or a combination of these things. If you eliminate pinched optics at the primary, and secondary, this (coma defects) are pretty much all you're left with.
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